Major, minor, patch content revisions

Any time a content field is changed, the change timestamp field is bumped, and a new revision may be created.

If the changed time is used as the "Last updated" date display and views sorting, minor changes could be shown before major ones.

For example, if I expand on a stub article, that would be a major change and I'd want to bump it to the top of the list. However, if I notice a typo, I'd like to be able to correct it without reordering.